THE ORGANS OF SPECIAI. SENSE 563 



epithelial elements, rod visual cells and cone visual cells. These, with 

 sup])orting connective tissue, constitute the layer of rods and cones 

 and the outer nuclear layer, the separation into sub-layers being due 

 to the sharp demarcation between the nucleated and non-nucleated 

 parts of the cells, and the separation of the two parts by the per- 

 forated outer limiting membrane. 



The rod visual cell (Fig. 373, A, 4) consists of rod, rud-librc, and 

 nucleus. The rod (Fig. 373, yl, 3) is a cylinder from 30 to 40," in 

 length and about 2/« in diameter. It is divisible into an outer clear 

 portion, which contains the so-called "visual purple" and an inner 

 granular portion. At the outer end of the latter is a fibrillated ellip- 

 soidal body, much more distinct in some of the lower animals, the 

 ellipsoid of Krause. At its inner end the rod tapers down to a line 

 fibre, the rod fibre, which passes through a perforation in the outer 

 limiting membrane into the outer nuclear layer, where it expands and 

 contains the nucleus of the rod visual cell. These nuclei are situated 

 at various levels in the fibre and constitute the most conspicuous 

 element of the outer nuclear layer (Fig. 373, B, 4). 



The cone visual cell (Fig. 373, A, 5,6) consists of cone, cone-fibre, 

 and nucleus. The cone (Fig. 373, ^, 5) is shorter and broader than 

 the rod , and like the latter is divisible into two parts. The outer part 

 is short, clear, and tapering, the inner part broad and granular, and 

 like the rod contains a fibrillated ellipsoid body. The cone fibre 

 (Fig. 2>73, A, 6) is much broader than the rod fibre, passes completely 

 through the outer nuclear layer and ends in an expansion at the mar- 

 gin of the outer molecular layer. The nucleus of the cone cell 

 usually lies just beneath the outer limiting membrane. 



The remaining layers of the retina must be considered in relation 

 on the one hand to the neuro-epithelium, on the other to the optic 

 nerve. The inner nuclear layer (Fig. 373, B, 6) and the layer of nerve 

 cells (Fig. 373, B, 8) are composed largely of nerve-cell bodies, while 

 the two molecular layers (Fig. 373, B, 5, 7) are formed mainly of the 

 ramifications of the processes of these cells. In the inner nuclear 

 layer are two kinds of nerve elements, rod bipolar cells and cone bipolar 

 cells. The bodies of these cells with their large nuclei form the bulk 

 of this layer. From the rod bipolars (Fig. 373, yl, 20) processes 

 {dendrites) pass outward to ramify in the outer molecular layer 

 around the terminations of the rod fibres. From the cone bipolars 

 (Fig. 373, A, 7) similar processes {dendrites) extend into the outer 

 molecular layer where they ramify around the terminations of the 



